In recent years, camera modules for taking photos have begun to be incorporated in mobile terminals such as mobile phones and lap-top computers. Downsizing the camera modules is a prerequisite for enhancing the portability of these apparatuses. The camera module operates with an image pickup device such as a CCD (Charged Coupled Device) or a CMOS (Complementary Metal Oxide Semiconductor). Recently, a pixel having the size of approximately a few micrometers has become commercially feasible, and an image pickup device with high resolution and a compact size can now be mass manufactured and marketed. This is accelerating the demand for downsizing of image pick-up lens systems so that they are able to be suitably used with miniaturized image pickup devices. It is also increasing expectations of cost reductions in image pick-up lens systems, commensurate with the lower costs enjoyed by modern image pickup devices. All in all, an image pick-up lens system needs to satisfy the oft-conflicting requirements of compactness, low cost, lightweight, and excellent optical performance.
A typical image pick-up lens system of a camera includes, from an object side to an image side, a biconvex first lens, a biconcave second lens, a meniscus-shaped third lens, and a biconvex fourth lens. All of the four lenses are spherical lenses made from glass materials. Therefore, all the lenses can be produced readily. However, the lenses are made from glass materials, which runs counter to the requirement of lightweight. In addition, shock resistances of glass materials are relatively low. If the camera falls off, the lenses in the lens system are liable to be damaged. Furthermore, the employment of spherical lenses always leads to spherical aberration, which goes against the requirement of excellent optical performance.
What is needed, therefore, a image pick-up lens system which has high shock resistance and lightweight is desired.